Refrigeration system with phase change material

ABSTRACT

A refrigerant system includes a vapor/compression refrigerant circulation loop ( 100 ) comprising a compressor ( 110 ), a refrigerant side of a heat exchanger condenser ( 120 ), an expansion device ( 130 ), and a refrigerant side of a heat exchanger evaporator ( 140 ), connected by conduit ( 143,115,125,135 ) in a closed circulation loop having refrigerant disposed therein. The refrigerant system also includes a flow path( 145 ) for a conditioned fluid across a conditioned fluid side of the heat exchanger evaporator and/or a conditioned fluid side of the heat exchanger condenser. During operation, this flow path can provide thermal communication between the refrigerant and the conditioned fluid. The refrigerant system also includes a phase change material (PCM), a thermal flow path between the fluid and the PCM, which provides thermal communication between the fluid and the PCM, and a thermal flow path between the refrigerant and the PCM, which provides thermal communication between the refrigerant and the PCM.

BACKGROUND OF THE INVENTION

The subject matter disclosed herein generally relates to refrigeration systems and their operation.

Refrigeration systems are known in the HVAC&R (heating, ventilation, air conditioning and refrigeration) art, and operate to compress and circulate a refrigerant throughout a closed-loop heat transfer fluid circuit connecting a plurality of components, to transfer heat away from and/or to a fluid such as air in a climate controlled space as with an air condition system, or alternatively to a secondary heat transfer fluid to be delivered to a climate-controlled space. In a basic refrigerant system, heat transfer fluid is compressed in a compressor from a lower to a higher pressure and delivered to a downstream heat rejection heat exchanger, commonly referred to as a condenser for applications where the fluid is sub-critical and the heat rejection heat exchanger also serves to condense heat transfer fluid from a gas state to a liquid state. From the heat rejection heat exchanger, where heat is typically transferred from the heat transfer fluid to ambient environment, high-pressure refrigerant flows to an expansion device where it is expanded to a lower pressure and temperature and then is routed to an evaporator, where heat transfer fluid cools the air in the conditioned environment or the secondary heat transfer fluid. From the evaporator, refrigerant is returned to the compressor. One common example of refrigerant systems is an air conditioning system, which operates to condition (cool and often dehumidify) air to be delivered into a climate-controlled zone or space. Other examples may include refrigeration systems for various applications requiring refrigerated environments.

Phase change materials (PCM's) have been used in refrigerant systems to act as a thermal buffer during thermal transfer operations, to shift thermal loads between peak and low demand cycles such as caused by diurnal outdoor temperature cycles, and/or to provide rapid responsive performance during peak system loads such as during the period immediately following loading of ambient temperature goods or items into a refrigerated space. For large commercial air conditioning systems, PCM systems, including phase change slurry (PCS) systems can be used to boost energy efficiency by load shifting operation by using cold thermal energy from the PCS during the daytime without turning on chiller compressors or the chiller unit, which was stored during the night when the equipment coefficient of performance (COP) is much higher. For small systems such as residential and light commercial air conditioning systems, since they are mostly cost sensitive and operated differently from the large systems, the application of PCMs for energy savings is not as straightforward. However, load shifting operation within the diurnal cycle of ambient temperature can still bring energy savings for systems integrated with PCMs. Other detailed benefits of PCM for small systems are explained next.

Most small air conditioning systems such as residential and light commercial air conditioning systems are sized to meet a maximum heat load of a house and building. These systems are rarely required to work at the maximum capacity conditions continuously over extended periods. They work mostly at partial load conditions, i.e. less than maximum capacity is required. The simplest way to meet a partial load condition is to turn the system on and off through a two-position control strategy by a thermostat. Another way is to use a Variable Frequency Drive (VFD) to control the compressor speed or a variable capacity compressor to meet the partial load operation. The first type of control, depending on the actual load dynamics, tends to result in energy efficiency degradation (cyclic loss) over an on/off cycle than that at the steady state. However, it is very simple and economical. The second approach is more expensive and adds some electrical loss from the power electronics.

With respect to efficiency degradation caused by on/off cycling of an air conditioning system, one of the factors is the elevated pressure lift the compressor sees over a whole cycle compared with that from a steady state operation. Such elevation of pressure lift is mainly caused by the nonlinear response of the metal temperature of the evaporator during compressor off cycle, i.e. a convex up curve, which indicates a heating up of the metal from the warm room air, and a concave down curve during the compressor on cycle, which indicates a cooling down of the metal from evaporation of the refrigerant. Depending on the length of the on or off interval, the average surface temperature can be different. The surface temperature determines the refrigerant pressure inside the tubes. PCM's thermally coupled with the refrigerant flow can change the thermal capacity of the evaporator, therefore, the dynamic responses of the evaporator by simply having the metal surface temperature following the PCMs temperature profile (a near constant value during phase change).

Many uses of PCM's in refrigerant systems have utilized serial thermal connections between the PCM, refrigerant, and the conditioned fluid, where the conditioned fluid can be the air in a conditioned space or water that is chilled (i.e., conditioned) and routed through a heat exchanger to cool air in the conditioned space. Examples of serial thermal connections include refrigerant

PCM

conditioned fluid, see, e.g., US 2011/0061410, US 2011/0048058, and/or US 2009/0205345. Such systems, although often effective, can be subject to limitations on heat transfer rates and/or thermal efficiency where the thermal throughput of the entire path is only as high as the most rate-limited thermal connection in the series.

BRIEF DESCRIPTION OF THE INVENTION

According to one aspect of the invention, a refrigerant system includes a vapor/compression refrigerant circulation loop comprising a compressor, a refrigerant side of a heat exchanger condenser, an expansion device, and a refrigerant side of a heat exchanger evaporator, connected by conduit in a closed circulation loop having refrigerant disposed therein. The refrigerant system also includes a flow path for a conditioned fluid across a conditioned fluid side of the heat exchanger evaporator and/or a conditioned fluid side of the heat exchanger condenser. During operation, this flow path can provide thermal communication between the refrigerant and the conditioned fluid. The refrigerant system also includes a phase change material (PCM) which can be disposed in a PCM thermal storage device, a thermal flow path between the fluid and the phase change material, which provides thermal communication between the fluid and the phase change material, and a thermal flow path between the refrigerant and the phase change material, which provides thermal communication between the refrigerant and the phase change material. During operation, the refrigerant system is able to provide simultaneous thermal communication between the conditioned fluid and the refrigerant, between the conditioned fluid and the phase change material, and between the refrigerant and the phase change material. Thus, this integration of PCM with a refrigerant system can provide parallel thermal flow paths during operation between the conditioned fluid, the refrigerant, and the phase change material.

In some aspects of the invention, the PCM thermal storage device is a receptacle disposed in the conditioned fluid flow path, containing a phase change material. In some aspects of the invention, the PCM thermal storage device is a receptacle disposed in the conditioned fluid flow path, containing a heat transfer fluid (HTF) that circulates between the receptacle and a PCM storage reservoir where it exchanges heat with a phase change material. In some aspects of the invention, the PCM thermal storage device is a receptacle disposed in the conditioned fluid flow path, containing a slurry of an encapsulated phase change material in a heat transfer fluid (a phase change material slurry or PCMS) that circulates between the receptacle and a PCMS storage reservoir. The HTF or PCMS in this system are collectively referred to herein as “PCM fluids”.

In a further aspect of the invention, a method of operating the above-described refrigerant system comprises operating the vapor/compression refrigerant circulation loop while flowing the conditioned fluid across the conditioned fluid side of the heat exchanger evaporator or across the conditioned fluid side of the heat exchanger condenser to transfer heat between the fluid and the refrigerant, while simultaneously flowing the conditioned fluid across the receptacle to transfer heat between the fluid and the phase change material.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a block schematic diagram depicting an exemplary embodiment of a refrigeration system as described herein; and

FIGS. 2A and 2B depict top and side views of an exemplary heat exchanger evaporator unit used in a refrigeration system as described herein.

DETAILED DESCRIPTION OF THE INVENTION

An exemplary refrigerant system is shown in block diagram form in FIG. 1. As shown in FIG. 1, a compressor 110 in refrigerant circulation loop 100 pressurizes a refrigerant (not shown) in its gaseous state, which both heats the refrigerant and provides pressure to circulate it throughout the system. The hot pressurized gaseous refrigerant exiting from the compressor 110 flows through conduit 115 to heat exchanger condenser 120, which functions as a heat exchanger to transfer heat from the refrigerant to the surrounding environment, such as to air circulation 122 blown by a fan (not shown) across the heat exchanger condenser 120. The hot refrigerant condenses in the heat exchanger condenser 120 to a pressurized moderate temperature liquid. The liquid refrigerant exiting from the condenser 120 flows through conduit 125 to expansion device 130, where the pressure is reduced. The reduced pressure liquid refrigerant exiting the expansion device 130 flows through conduit 135 to the heat exchanger evaporator 140, from which it flows through conduit 143 to the inlet of compressor 110, thus completing the loop. The heat exchanger evaporator 140 functions as a heat exchanger to absorb heat from (i.e., cool or condition) a fluid such as air in a space to be air conditioned or refrigerated depicted in FIG. 1 as air flow 142 that flows across the heat exchanger evaporator 140. Alternatively, the fluid being cooled by heat exchanger evaporator can be a heat transfer fluid like water, with water chilled by the heat exchanger evaporator 140 flowing in a secondary heat transfer circuit through another heat exchanger (not shown) across which air to be conditioned or refrigerated is passed. Additionally, as is known in the art, the system can also be operated in heat pump mode using a standard multiport switching valve to reverse refrigerant flow direction and the function of the condenser and evaporator heat exchangers, i.e. the condenser in cooling mode being evaporator in heat pump mode and the evaporator in cooling mode being the condenser in heat pump mode.

Further details of heat exchanger/evaporator 140 are shown in FIGS. 2A and 2B, which depict a top view and side view, respectively, of the heat exchanger evaporator 140. As shown in FIGS. 2A and 2B, heat exchanger/evaporator 140 has a housing 205 having fins 210 disposed therein as can be typically found in a fin tube heat exchanger. In addition to providing a mounting structure for the fins and tubes in the heat exchanger/evaporator 140, the housing 205 also provides an enclosure for a flow path of conditioned fluid 235, which can be air or water, for example, as described above. Refrigerant tubes 215 are disposed in the housing 205 intersecting and in thermal contact with the fins 210. Note that, as used herein, the plural “tubes” can refer to multiple tubes routed in parallel through the heat exchanger/evaporator 140 or a single tube with multiple tube length passes through the heat exchanger/evaporator 140. For ease of illustration, refrigerant tubes 215 are not shown in FIG. 2A (they are shown in FIG. 2B), but are routed through the housing 205 and the fins 210 in similar fashion as the PCM tubes 220 that are shown in FIG. 2A. The routing of both the PCM tubes 220 and the refrigerant tubes 215 through the same rack of fins 210 provides a conductive thermal connection between the refrigerant and the phase change material through the tube walls and the fins.

Referring now to FIGS. 1 and 2A, a PCM fluid such as a phase change slurry is disposed in PCM tubes 220 and can circulate in PCM loop 145 between the heat exchanger/evaporator 140 and a PCM reservoir 150. Specifically, the PCM fluid is drawn from PCM reservoir 150 by pump 155 through conduit 160, valve 165, and conduit 170, and then through conduit 175 to PCM tube inlet 225. The PCM fluid flows through PCM tubes 220 and then exits the heat exchanger/evaporator 140 through PCM tube outlet 230, from which it flows through conduit 180, valve 185, and conduit 190 to return to the PCM reservoir 150. In an alternative embodiment (not shown), PCM tubes 220 are not connected to a PCM fluid loop 145, and the phase change material is instead statically contained in the PCM tubes 220. In the exemplary system of FIG. 1, the rate of heat transfer to or from the PCM can be controlled by controlling the speed of the pump 155 and/or the position of the valve 160 and valve 185. In some embodiments (not shown), the PCM fluid can be routed through or across the heat exchanger evaporator 140 or the heat exchanger condenser 120, either of which can be configured as a multi-sided heat exchanger to accommodate refrigerant, PCM fluid, and conditioned fluid and provide parallel thermal flow paths. More specifically, the PCM fluid can be routed to the heat exchanger condenser 120, now used as a heat sink.

During one mode of operation, while compressor 110 is on, the PCM fluid can be cooled down to or below the solidification temperature of the PCM, for example, <5° C., and cooling capacity is stored into the PCM slurry or PCM inside the PCM reservoir 150. The temperature of the PCM slurry stays at the solidification temperature until all of the PCM inside the PCM reservoir 150 solidifies. During this time, the refrigerant in refrigerant tubes 205 also cools the conditioned fluid (e.g., air) while simultaneously transferring heat from the PCM fluid, thus storing cooling capacity in the PCM reservoir 150. This parallel thermal coupling between the conditioned fluid and the PCM and refrigerant offers an alternative thermodynamic efficiency profile to the traditionally used refrigerant-PCM-air serial coupling while providing the benefits of PCM thermal energy storage such as managing diurnal external temperature cycles and refrigeration load variability.

The PCM fluid can be a combination of heat transfer fluid and encapsulated PCM particles or a solution of organic or inorganic salts that can form crystal slurry when cooled down to below crystallization temperatures. In the case of a slurry of encapsulated PCM particles (e.g., ranging from 10 microns to 200 microns), inside which a solid high thermal conductivity material (e.g., fused silica, vitreous silica) can be blended with an organic PCM during the encapsulation process to provide a PCM slurry having enhanced thermal conductivity.

In addition to the above-described mode of operation, other modes of operation can be readily provided by the refrigerant system. For example, in another mode of operation, pre-stored cooling capacity stored in the PCM storage reservoir can be used to increase the effective system cooling capacity during temporary periods of high demand such as following loading operations into or out of a refrigerated truck or other refrigerated space. When the PCM fluid pump and the compressor run at the same time, the expanded refrigerant and the cooling from the PCM fluid circulating through the PCM storage reservoir and the evaporator heat exchanger can cool the load at the same time or the PCM fluid flowing through the condenser can function as a heat sink. In yet another mode of operation, pre-stored cooling capacity in the PCM can be used to provide an efficient alternative to system control utilizing a variable speed compressor. In this mode, the PCM can provide variable cooling capacity to supplement operation of the refrigerant loop with the compressor operating at a fixed speed, thus avoiding the inefficiencies of on/off control without the use of a variable speed compressor. When both the PCM fluid pump and compressor run at speed, the storage is charged. The charged cooling capacity can be discharged at a partial load while the compressor is turned off.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A refrigeration system for cooling a fluid, comprising: a vapor/compression refrigerant circulation loop comprising a compressor, a refrigerant side of a heat exchanger condenser, an expansion device, and a refrigerant side of a heat exchanger evaporator, connected by conduit in a closed circulation loop having refrigerant disposed therein; a fluid flow path for said fluid through or across the heat exchanger evaporator or the heat exchanger condenser, which provides thermal communication between the fluid and the refrigerant; a phase change material; a thermal flow path between the fluid and the phase change material, which provides thermal communication between the fluid and the phase change material; and a thermal flow path between the refrigerant and the phase change material, which provides thermal communication between the refrigerant and the phase change material; and


2. The refrigerant system of claim 1, wherein the fluid is air.
 3. The refrigerant system of claim 1, wherein the fluid is liquid water.
 4. The refrigerant system of any of claims 14, wherein the phase change material comprises water.
 5. The refrigerant system of claim 1, wherein the phase change material comprises a paraffin wax and/or a fatty acid.
 6. The refrigerant system of claim 1, further comprising a PCM receptacle that contains a PCM fluid that comprises or is in thermal communication with the phase change material.
 7. The refrigerant system of claim 6, wherein the thermal flow path between the refrigerant and the PCM fluid comprises a thermally conductive material connecting the PCM receptacle and the heat exchanger evaporator or the heat exchanger condenser.
 8. The refrigerant system of claim 6, wherein the heat exchanger evaporator or the heat exchanger condenser and the PCM receptacle are disposed within a housing through which the fluid flow path is directed through or across the heat exchanger evaporator or the heat exchanger condenser and the PCM receptacle
 9. The refrigerant system of claim 8, wherein the heat exchanger evaporator comprises a plurality of refrigerant tubes disposed in the flow path of the fluid within the housing, the PCM receptacle comprises a plurality of tubes disposed in the flow path of the fluid within the housing, and the thermal flow path between the refrigerant and the PCM fluid comprises a plurality of fins in contact with both the refrigerant tubes and the PCM receptacle tubes.
 10. The refrigerant system of claim 6, wherein the phase change material is statically contained in the PCM receptacle.
 11. The refrigerant system of claim 6, further comprising a reservoir that comprises a phase change material, remote from the PCM receptacle, and a circulatory conduit for PCM fluid between the PCM receptacle and the reservoir.
 12. The refrigerant system of claim 11, further comprising a circulatory conduit for PCM fluid between the reservoir and the heat exchanger condenser or the heat exchanger evaporator, which provides thermal communication between the refrigerant and the phase change material.
 13. The refrigerant system of claim 1, wherein the PCM fluid comprises a phase change material encapsulated in a slurry of capsules dispersed in a heat transfer fluid.
 14. A method of operating the refrigerant system of claim 1, comprising operating the vapor/compression refrigerant circulation loop while simultaneously flowing the fluid through or across the heat exchanger evaporator or the heat exchanger condenser while simultaneously flowing the fluid across the PCM receptacle.
 15. A method of operating the refrigerant system of claim 11, comprising operating the vapor/compression refrigerant circulation loop while simultaneously flowing the fluid through or across the heat exchanger evaporator or the heat exchanger condenser, flowing the fluid across the PCM receptacle, and circulating PCM fluid between the PCM receptacle and the reservoir.
 16. The method of claim 14, wherein the fluid flows through or across the heat exchanger evaporator.
 17. A method of operating the refrigerant system of claim 12, comprising operating the vapor/compression refrigerant circulation loop while simultaneously flowing the fluid through or across the heat exchanger evaporator or the heat exchanger condenser, flowing the fluid across the PCM receptacle, circulating PCM fluid between the PCM receptacle and the reservoir, and circulating PCM fluid between the reservoir and the heat exchanger evaporator or the heat exchanger condenser.
 18. The method of claim 17, wherein the fluid flows through or across the heat exchanger evaporator and the PCM fluid circulates between the reservoir and the PCM fluid circulates between the reservoir and the heat exchanger condenser and between the reservoir and the PCM receptacle. 